Case study: 07 June 2018

In the afternoon of 07 June 2018, severe storms affected the eastern São Paulo state and southern Minas Gerais state. The storms formed under a very intense upper-level jet, which caused deep-layer shear of more than 25 m/s in some locations. The thermodynamic instability was not high, but sufficient to promote convective updrafts along a stationary front, which in turn became severe due to the very high wind shear.

The GFS MUCAPE fields are shown below. Eastern São Paulo had nearly 1000 J/kg of CAPE associated with 25+ m/s of 1000-500-hPa wind shear magnitude. The São Paulo soundings of 07/06 1200 UTC and 08/06 0000 UTC show very strong winds from low to upper levels of the troposphere, and moderate instability in the midlevels. The storm-relative helicity values below -100 m²/s² are very favorable for rotating, long-lived storms, as the ones observed in the region.

 

The storm that moved across São Luiz do Paraitinga-SP, causing 2+-cm hail and high winds is shown below. The 35-dBZ echo top peaked at more than 15 km height, which is an extreme value.

 

Case study: 19 May 2018

A strong cold front was responsible for the formation of a squall line over São Paulo state on the morning of 19 May 2018. This system caused severe winds in the city of São Paulo which were accompanied by fallen trees and power lines (https://g1.globo.com/sp/sao-paulo/noticia/bombeiros-recebem-mais-de-30-chamados-para-quedas-de-arvores-em-sp-apos-chuva-forte-com-rajada-de-vento.ghtml). The strongest wind gust was 93 km/h in the Guarulhos airport.

The strong cold front approaching São Paulo state on the morning of 19 caused strong low-level northwesterly flow over the area, which contributed to relatively high wind shear and transport of warm/moist air ahead of the squall line. The GFS analysis show that CAPE was not very high, but sufficient to produce severe weather given the wind shear greater than 20 m/s.

The 19 May 2018 12 UTC sounding of São Paulo depicts a relatively unstable lower troposphere, with warm air under relatively colder air in the midtroposphere. The winds were strong as well, with 40 kt just above the surface. This pattern of strong instability and high winds in the boundary layer favored severe wind gusts.

The radar imagery is shown below. A classic squall line is observed passing over São Paulo. The line caused very little precipitation.

Case study: 28-31 March 2018

Thunderstorms stroke São Paulo city in five consecutive days in the end of March 2018. Multiple flash floods and fallen trees were reported throughout the city (https://g1.globo.com/sp/sao-paulo/noticia/chuvas-causam-queda-de-89-arvores-em-sao-paulo-no-feriado.ghtml), causing chaos in the traffic.

A stationary trough was located over the Andes of northwestern Argentina and northern Chile. A low-pressure system was located over Paraná state and provided continuous northwesterly flow from the Amazon Basin towards São Paulo state.  In the afternoons, the moisture-rich environment was suitable for deep convection.

The sounding from 28 March through 31 March at 12 Z are shown below. The soundings are very similar, corroborating the stationary regime in which the storms occurred in these consecutive days. All soundings show very weak winds in the middle and lower troposphere, and a nearly moist-adiabatic profile. 

Some radar images are shown below to exemplify the storm modes observed in the region. The convection was not very intense nor organized, but the fact that it occurred in a city with poor drainage was the key factor.

Case study: 26 March 2018

The city of Jacareí, in the Paraíba Valley, reported flash floods on 26 March 2018. Several streets were flooded due to the high precipitation rates (https://g1.globo.com/sp/vale-do-paraiba-regiao/noticia/temporal-causa-alagamentos-e-agua-invade-creche-em-jacarei-sp.ghtml).

The São Paulo-SP 12 Z sounding depicts a moist layer at low levels and a temperature inversion at nearly 600 hPa. Above the inversion, the southerly winds are relatively strong. After strong radiative heating, the instability of this profile increased significantly.

The storm is shown below. It persisted over Jacareí for about 1:30h, and this slow movement was determinant in the flash flood. VIL values over Jacareí were greater than 40 mm for more than one hour.

VIL at 1950 Z 26/03/2018, São Roque radar. 

Reflectivity at 1950 Z 26/03/2018, São Roque radar.

Case study: 20 March 2018

The afternoon and night of 20 March 2018 were marked by heavy precipitation in the eastern half of São Paulo sate. Several cities reported damage due to fallen trees, flash flooding, and huge amounts of lightning. São José dos Campos reported small hail (https://g1.globo.com/sp/vale-do-paraiba-regiao/noticia/sao-jose-dos-campos-tem-chuva-de-granizo-na-primeira-tarde-do-outono.ghtml), Guaratinguetá had flash flooding (https://g1.globo.com/sp/vale-do-paraiba-regiao/noticia/temporal-provoca-alagamentos-em-casas-e-ruas-em-guaratingueta.ghtml) and in São Paulo 95 trees fell and 2 people died, along with the impressive amount of 14 thousand lightning strikes over the city (https://g1.globo.com/sp/sao-paulo/noticia/cerca-de-mil-pessoas-estao-fora-de-casa-apos-chuva-em-sp-62-arvores-e-14-mil-raios-cairam.ghtml).

The synoptic-scale environment was characterized by a trough in southern Brazil with attendant cold front over the coast of Paraná state. The cold front was not moving too fast north. However, there was an abrupt change in the lo-level wind direction over Paraná and São Paulo states, which directly increased low-level convergence over São Paulo. Below are the 18Z and 00Z 925-hPa specific humidity and winds, showing the change in wind direction over São Paulo state during the precipitation event.

 18Z:

 00Z:

Another feature that probably played a significant role was the upper-level jet located over southern Brazil, which moved about 200 km to the north along with the upper-level trough and surface front (below). São Paulo state was located in the equatorward entrance region of the jet, where ascent is enhanced by the ageostrophic circulations of the jet.

The radar imagery depicts de formation of several storms during the afternoon, and upscale growth in early night as ascent increased over the area. Some squall lines accompanied the precipitation area.

18Z:

20Z:

21Z:

Case study: 14 March 2018

Several cities in eastern São Paulo state were affected by heavy precipitation and severe winds on the afternoon of 14 March 2018. In Campinas, winds greater than 85 km/h were reported, resulting in fallen trees and damage to buildings (https://g1.globo.com/sp/campinas-regiao/noticia/temporal-em-campinas-e-valinhos-tem-vento-a-85-kmh-e-queda-de-arvores.ghtml).

The synoptic-scale flow in the mid/upper troposphere was characterized by a ridge north of São Paulo state, and strong zonal flow in the southern part of the state (500-hPa field below). There was no synoptic-scale forcing for ascent, but the strong zonal flow was important to the relatively high wind shear in the area.

 Fig: 500-hPa cyclonic vorticity (10^-5 s^-1, shaded), geopotential height (dam, black contours), temperature (°C, blue contours) and winds at 18Z 14 March 2018.

Moisture was not a problem in the area, where dewpoints reached more than 20 °C in several locations. The 850-hPa flow (below) shows moderate northwesterly flow in the region in association with specific humidity in the lowest 100 hPa of more than 15 g/kg, which characterizes a very moist (and buoyant) planetary boundary layer.

Fig: Specific humidity in the lowest 100 hPa (g/kg, shaded) and 850-hPa streamlines at 18Z 14 March 2018.

The 12Z São Paulo sounding (below) depicts a very unstable profile for the morning. With the expected diurnal heating, temperatures rose to more than 33°C, and CAPE increased to more than 2500 J/kg (second figure below). Also, the winds were relatively intense in the lower and mid troposphere, which favored severe storms.

Fig: Most unstable CAPE (J/kg, shaded) and 1000-500-hPa wind shear at 18Z 14 March 2018.

The storm that affected Campinas is shown below. The storm formed over the Campinas area at 00:40 UTC (first figure below) and caused intense precipitation. It soon evolved to a bow echo at 01:30 UTC (second figure below), which is generally associated with high winds.

Fig: São Roque reflectivity (dBZ) at 00:40Z and 01:30Z 15 March 2018.

The VIL was also high (> 40 mm) over Campinas as soon as the storm formed, and indicated a high-precipitation storm. The high precipitation was likely responsible for the bow echo formation as the cold pool spread over the surface.

Fig: São Roque VIL (mm) at 00:40Z 15 March 2018.

Case study: 05 March 2018

Some cities in the Paraiba river valley experienced strong winds in the afternoon of 05 March 2018. In São José dos Campos, the wind gusts peaked at 75 km/h, and nearly 100 trees fell, causing trouble to the traffic (https://g1.globo.com/sp/vale-do-paraiba-regiao/noticia/temporal-com-ventos-de-mais-de-75-kmh-derruba-cerca-de-100-arvores-e-deixa-bairros-sem-energia-em-sao-jose.ghtml).

The lower and middle troposphere had very weak winds on te 05/03 afternoon, with low-level winds from the southwest at no more than 10 m/s. The precipitable water ranged from 40 to 45 mm, which is typical at this time of the year. This very moist environment helped in the high precipitation caused by some storms in the region. The 925 hPa winds and specific humidity in the lowest 100 hPa (figure below) show the very moist air mass over the area and weak 925-hPa winds. However, the sea breeze at this time caused inland-directed winds, and enhanced low-level convergence over the Paraiba river valley at this time.

Specific humidity in the lowest 100 hPa and 925-hPa winds at 1800 UTC 05/03/2018.

The reflectivity core that passed over São José dos Campos is shown below. The highest reflectivity was nearly 55 dBZ at 1.0° elevation, which characterizes high amounts of hydrometeors. The most affected area was the south of the city, exactly where the convective core is located at 1920 UTC.

Refletividade (dBZ) no PPI de 1.0° às 1920 UTC do dia 05/03/2018.

O VIL mostra valores altíssimos (>50mm) sobre a região. Esses valores sõ condizentes com alta quantidade de hidrometeoros na tempestade. Devido aos fracos ventos na troposfera, pode-se concluir que a descida da precipitação nas correntes ascendentes foi o que mais contribuiu para causar ventos fortes em superfície.

 

Vertically Integrated Liquid (VIL; mm) no CAPPI de 3km às 1920 UTC do dia 05/03/2018.

Abaixo está o Echo-top de 35 dBZ, que mostra que refletividades de 35 dBZ foram observadas pelo radar em mais de 12 km de altura, ou seja, tratava-se de grande quantidade de gelo na alta troposfera. Essa é uma assinatura de severidade da tempestade.

 

 Echo-top de 35 dBZ (km) às 1920 UTC do dia 05/03/2018.

Case study: 26 February 2018

On 26 February 2018, the city of São Paulo was affected by heavy rain, which causes flash floods and traffic jams all over the metropolitan region. Nearly 70 mm of precipitation fell in a few hours (https://g1.globo.com/sp/sao-paulo/noticia/chuva-deixa-zona-leste-de-sp-em-estado-de-atencao-para-alagamentos.ghtml).

Below is the specific humidity averaged over the first 100 hPa with 850-hPa streamlines. Convergence is observed along a windshift (trough) that extends from western São Paulo state towards the Atlantic Ocean, where convergence occurs. The 100-hPa specific humidity is maximized along the trough axis, where the highest precipitation occurred.



The 12Z sounding at São Paulo depicted a warming boundary layer overrunned by several small inversions and steep lapse rate layers. The wind profile was relatively weak, but some anticyclonic turning with height was observed in the lowest levels.



The radar imagery between 1800 and 2000 UTC is shown below. Some thunderstorms formed over São Paulo near a southeast-northwest-oriented band of precipitation farther north, and causes intense precipitation at 1800 UTC. Then, the precipitation stalled over the city causing the high rainfall totals and hazardous weather.

1800 UTC:

 
1900 UTC:



 
2000 UTC:





The accumulated precipitation estimated by radar is shown below. Some points of São Paulo had 60-70 mm according to radar estimates, which nicely matches the observed precipitation.

Case study: 24 January 2018

Several severe storms occurred in central São Paulo state on 24 January 2018. The cities of Araraquara (https://g1.globo.com/sp/sao-carlos-regiao/noticia/chuva-forte-em-araraquara-sp-derruba-arvores-e-destelha-casas-veja-video.ghtml) and Bragança Paulista (https://g1.globo.com/sp/vale-do-paraiba-regiao/noticia/telhado-de-universidade-cai-durante-temporal-em-braganca-paulista-veja.ghtml) were the most affected.

The environment was characterized by relatively high geopotential heights in the mid and upper troposphere (Fig. 1), with anticyclonic circulation in the flow. This prevented strong convective overturning and guaranteed a low number of storms in the region, some of which were severe. São Paulo sounding was not launched in this day, unfortunately, but GFS estimative of CAPE ranged between 2000 and 2500 J/kg at 18Z.

Fig. 1: Wind speed (m/s, shaded), wind barbs (kt) and geopotential height (dam, black contours) at 18Z 24 January 2018.

Among the most impressive characteristics of the storms that affected Araraquara and Bragança Paulista was the very high value of vertically integrated liquid (VIL). Figs. 2 and 3 shows VIL values over 50 mm in both storms, which was likely associated with high amounts of hidrometeors (mainly hail) in the troposphere.

 Fig. 2: VIL derived from São Roque S-band radar at 18:20Z 24 January 2018.

 Fig. 3: VIL derived from São Roque S-band radar at 18:50Z 24 January 2018.

Case study: 20-21 January 2018

Some cities in eastern São Paulo state were affected by flash floods in 20 and 21 January 2018. On 20, Indaiatuba registered heavy rain and some river rise over the banks (https://g1.globo.com/sp/campinas-regiao/noticia/temporal-em-indaiatuba-desaloja-familias-derruba-muro-e-arvores-e-deixa-carro-em-pe-apos-ser-arrastado-por-enxurrada.ghtml). Some neighborhoods of São Paulo were also flooded due to large amounts of precipitation in a short period of time (https://g1.globo.com/sp/sao-paulo/noticia/enxurrada-arrasta-carros-e-pedestre-no-beco-do-batman-em-sp.ghtml and http://www1.folha.uol.com.br/cotidiano/2018/01/1952205-chuva-forte-derruba-arvores-e-alaga-ruas-na-zona-oeste-de-sao-paulo.shtml).

The synoptic environment will be accessed  using the 0000 UTC 21 January GFS analysis. The precipitable water was nearly 50 mm in the region, and the 700-hPa winds were very quiescent, which favors slow-moving convective systems and heavy precipitation.

Fig. 1: Precipitable water (mm, shaded), and 700-hPa geopotential height (dam) and wind at 00Z 21 Jan 2018.

The sounding of São Paulo at 00Z 21 Jan 2018 shows a moist profile throughout the troposphere and very light winds. This sounding suggests the storms formed in the area had no organization due to the absence of wind shear, but posed a threat to flash flood because of the very slow storm motion speed and high precipitable water.

Fig. 2: São Paulo sounding at 00Z 21 Jan 2018.

The storm occurred in  Indaiatuba on 20 Jan is show in Fig. 3 at peak intensity over the city. The storm system was moving east-southeastward and formed a line just before crossing the city. The sudden organization of the storm system seems to have contributed to the increase in VIL (Fig. 4), which reached more than 30 mm.

Fig. 3: Reflectivity of São Roque radar at 20:00Z 20 Jan 2018.

Fig. 4: Vertically integrated Liquid of São Roque radar at 20:00Z 20 Jan 2018.

The other intense storm that affected Sâo Paulo on 21 Jan is shown in Fig. 5. Curiously, VIL was very low (< 15 mm) in this area, but this storm caused flash flooding. Even though VIL was low, the storm was nearly stationary. However, this case is an example of challenging nowcasting scenario, with low predictability.

Fig. 5: Reflectivity of São Roque radar at 18:40Z 21 Jan 2018.

Case study: 18 January 2018

     In the afternoon of 18 January, a severe thunderstorm caused wind damage in São José dos Campos-SP. Several trees and power lines were downed by the storm (https://g1.globo.com/sp/vale-do-paraiba-regiao/noticia/chuva-e-vento-derrubam-arvores-nas-zonas-norte-e-sudeste-de-sao-jose.ghtml). High winds were also observed in Botucatu-SP (https://g1.globo.com/sp/bauru-marilia/noticia/temporal-derruba-arvores-e-prejudica-fornecimento-de-energia-em-botucatu.ghtml).

     The São Paulo-SP 12Z sounding shows an unstable lower troposphere, with CAPE over 800 J/kg in the morning and 20°C of dewpoint temperature, quite high for São Paulo. After radiative heating, the CAPE would increase to more than 2000 J/kg. Also, the winds in the 1000-500-hPa layer are relatively strong for mid summer, which helped to organize the storms in the region.

 Fig. 1: 12Z 18 January 2018 São Paulo sounding.

     Storm initiation took place over São José dos Campos at nearly 18Z along a north-south line. The storm attained high reflectivity (Fig. 2) over the city and remained stationary for about 1 hour.

Fig. 2: São Roque radar reflectivity at 18:50Z 18 January 2018.

     The analysis of VIL and Echo Top (Fig. 3) at 18:50Z evidence that the storm presented severe characteristics. The VIL values are higher than 35 mm at the center of the storm, which possibly indicates large amounts of hail. 35-dBZ echoes were observed at altitudes higher than 10 km, and supports the fact that the storm contained high amounts of hail in its core.

Fig. 3: São Roque radar (a) VIL and (b) 35-dBZ Echo top at 18:50Z 18 January 2018.

Case study: 25 December 2017

In the Christmas afternoon, a squall line (Fig. 1) moved through central São Paulo State and caused intense precipitation over Campinas-SP region (https://g1.globo.com/sp/campinas-regiao/noticia/campinas-tem-temporal-com-ventos-de-mais-de-80-kmh-e-alagamentos.ghtml). Several parts of Campinas were rapidly flooded due to high precipitation rates. According to Cepagri, the measured precipitation rate was nearly 20 mm/h, which is sufficient to cause flash flooding. The accumulated precipitation is shown in Fig. 2. The precipitation over Campinas (20-25 mm) occurred in only 1-2 hours.

 Fig. 1:  Reflectivity at 1730 UTC 25 December 2017, X-band SOS Chuva radar at Campinas-SP.

 Fig. 2: Accumulated precipitation in the last 6 h at 2100 UTC 25 December 2017, estimated by the X-band SOS Chuva radar at Campinas-SP.

The synoptic-scale environment at 1800 UTC 25 December 2017 at 700 hPa (Fig. 3) was characterized by a high precipitable water corridor extending from the Amazon towards São Paulo State, in association with northwesterly flow in the lower levels. The precipitable water over the Campinas region ranged between 50 and 55 mm. Confluence was observed over São Paulo state between the 700-hPa ridge over Minas Gerais state and the cold-frontal trough over the Atlantic Ocean farther south.

 Fig. 3: GFS analysis of precipitable water (mm, shaded) and 700-hPa winds, geopotential height (dam, black contours) and temperture (°C, red contours) at 1800 UTC 25 December 2017.

 The 1200 UTC sounding at São Paulo-SP is shown in Fig. 4. The temperature profile was nearly moist adiabatic in the entire troposphere, but the radiative heating noted at surface increased the surface temperature and thermodynamic instability in the afternoon. CAPE values were around 1000 J/kg according to the GFS analysis. This setup of moderate instability, large-scale confluence, high precipitable water and moderate low-level winds favored the formation of moderately organized convection capable of heavy rain in the area.

 Fig. 4: 1200 UTC 25 December 2017 São Paulo-SP sounding.

Figure 5 shows the Vertically Integrated Liquid (VIL) before the squall line hit Campinas. VIL values were peaking at 20 mm in some areas within the squall line, which indicated the high potential for flash flooding approaching the Campinas area. This figure elucidates how a nowcasting system can be used to forecast high-impact events with several hours of leading time.

Fig. 5: Vertically integrated liquid at 1630 UTC 25 December 2017 generated by the X-band SOS Chuva radar at Campinas-SP.